Normal growth, differentiation, and survival in multicellular organisms requires a carefully regulated balance, or homeostasis, between the production and destruction of cells in tissues throughout the body. Cell division is a carefully coordinated process with numerous checkpoints and control mechanisms. These mechanisms are designed to regulate DNA replication and to prevent inappropriate or excessive proliferation. In contrast, apoptosis is a genetically controlled process by which unneeded or damaged cells are eliminated without causing the tissue destruction and inflammatory responses that are often associated with acute injury and necrosis.
A variety of ligands and their cellular receptors, enzymes, tumor suppressors, viral gene products, pharmacological agents, and inorganic ions have important positive or negative roles in regulating and implementing the apoptotic destruction of a cell. Although some specific components of the apoptotic pathway have been identified and characterized, many interactions between the proteins involved are undefined, leaving major aspects of the pathway unknown (Thompson, C. B. (1995) Science 267:1456-1462).
Mammalian cells in culture can be grown for only a limited number of cell divisions after which they cease proliferation and exhibit the morphological changes associated with cellular senescence. Evidence in support of a genetic determinant for aging has been obtained in various organisms. For instance, in the yeast Saccharomyces cerevisiae, the patterns of expression of certain genes change in a specific manner during the life span, and these changed patterns suggest that the aging process is subject to gene regulation.
Controlled expression of the transforming gene of Harvey murine sarcoma virus (v-Ha-ras) was found to extend yeast life span (as measured by the number of cell divisions) nearly two-fold (Jazwinski, S. M. et al. (1993) Adv. Exp. Med. Biol. 330: 45-53; Chen, J. B. et al. (1990) Mol. Microbiol. 4: 2081-2086). RAS1 and RAS2, which are yeast homologs of the v-Ha-ras oncogene, play central roles in the integration of cell growth and the cell cycle in yeast. The primary role of these RAS proteins in yeast is the GTP-dependent regulation of adenylate cyclase activity. Curiously, mutations in RAS1 and RAS2 have opposite effects on yeast life span. Sun, J. et al. (1994; J. Biol. Chem 269:18638-18645) observed that deletion of RAS1 lengthened life span while deletion of RAS2 decreased life span. Elevated expression of yeast RAS2 led to a 30% increase in life span and postponed the senescence-related increase in cell generation time seen during yeast aging. No life span extension was observed by overexpression of RAS1, although both RAS1 and RAS2 mRNA and protein levels declined with replicative age.
D'mello, N. P. et al. (1994; J. Biol. Chem. 269:15451-15459) isolated a yeast gene denoted longevity-assurance gene-1 (LAG1). LAG1 expression is highest in young cells and decreases as yeast cells age. The predicted translation product is a 411 amino acid protein (denoted LAG1p or LAP1) which contains clusters of potential phosphorylation sites near the N- and C-termini and multiple potential transmembrane domains. Deletion of LAG1 resulted in a significant increase in the mean and maximum number of cell divisions; the mean life span increased from 17 to 25 cell divisions, and the maximum life span increased from 25 to 37 cell divisions. D'mello et al. (supra) proposed that the expression of LAG1 in young yeast cells may set a threshold which determines the extent to which the cells can divide. Deletion of LAG1 apparently alters this threshold, and a different gene(s) may then become the limiting factor in longevity (D'mello et al., supra). LAC1, described as a virtual copy of LAG1, is found on a different yeast chromosome and also alters longevity (Jazwinski, S. M. (1996) Science 273:54-59; Kirchman, P. A. and Jazwinski, S. M., unpublished).
Aberrant regulation of cellular homeostasis is a significant factor in the pathogenesis of human disease. For example, inappropriate cell survival can cause or contribute to diseases such as cancer, autoimmune diseases, and inflammation. In contrast, increased apoptosis can cause or contribute to immunodeficiency diseases such as AIDS, neurodegenerative disorders including Alzheimer's disease, and myelodysplastic syndromes (Thompson, C. B. (1995) Science 267:1456-1462).
Furthermore, numerous diseases and disorders are associated with aging. Diseases which show age-dependent onset of symptoms include Alzheimer's disease, Pick's disease, Huntington's disease, Parkinson's disease, adult onset myotonic dystrophy, multiple sclerosis, adult onset leukodystrophy, diabetes mellitus, arteriosclerosis, and cancer.
Patients who suffer from premature aging syndromes exhibit numerous defects associated with more advanced age groups. Symptoms of Werner's syndrome include scleroderma-like skin changes, cataract, subcutaneous calcification, premature arteriosclerosis, and diabetes mellitus. A striking aspect of Werner's syndrome, presumably arising from the same genetic defect, is a dramatic shortening of the replicative life-span of dermal fibroblasts in vitro (Faragher, R. G. et al. (1993) Proc. Natl. Acad. Sci. USA 90:12030-12034). Fibroblasts from Werner's syndrome patients exit irreversibly from the cell cycle at a faster rate than do normal cells, although they generally start off with a good replicative ability. Faragher, R. G. et al (supra) proposed that the Werner syndrome gene is a "counting" gene which controls the number of times that human cells are able to divide before terminal differentiation.
The discovery of two new human longevity-assurance protein homologs and the polynucleotides encoding them satisfies a need in the art by providing new compositions which are useful in the diagnosis, prevention and treatment of disorders associated with aberrant regulation of cellular homeostasis and disorders associated with aging.